(1) Field of the Invention
The present invention relates to a mounting technique for fixing mirrors that are used for constructing an optical system. In particular, the invention relates to a mirror fixing method for preventing stress distortion of mirror surface due to the fixing, and to an optical apparatus using the method.
(2) Related Art
Normally, in order to construct an optical system, members such as a light source, lenses and mirrors are used to change an optical path or to collect lights. For example, as shown in FIG. 8, a mirror part 100 is sometimes used for changing a propagation direction of light emitted from a light source by 90°. In this mirror part 100, typically a mirror 101 with a mirror surface formed by vapor deposition of metal or the like on the surface of a base material made of glass, plastic, or the like, is fixed on a jig 102 for use. As a conventional method of fixing the mirror 101 on the jig 102, there is known a method in which an adhesive is applied on the back of the mirror 101, which is then adhered at a desired position of the jig 102. Moreover, for example, as shown in FIG. 10, there is also applied a method where the mirror 101 is inserted into the jig 102 with a threaded type fixing ring 103.
The mirror 101 fixed to the jig 102 as described above, is used as a part of an optical system such as a variable wavelength dispersion compensator (to be referred to as a VIPA-VDC) which uses a VIPA (abbreviation of Virtually Imaged Phased Array, a device that branches for each wavelength, an optical signal that is a combination of more than one wavelength) as disclosed in Japanese National Publication Nos. 2000-511655 and 2002-514323 of PCT Applications previously filed by the present applicant. FIG. 11 is a perspective view showing an outline of the abovementioned VIPA-VDC. This VIPA-VDC has a configuration where, for example, a mirror assembly 110 in which an aspherical mirror 112 fixed to an L-shaped jig 111 is installed on a movable stage 113, is combined with a VIPA assembly 120 in which an optical fiber 121, collimator lenses 122 and 123, a VIPA 124, and a collimator lens 125 are arranged in that order. In this VIPA-VDC, light emitted from the optical fiber 121 is input to the VIPA 124 via the collimator lenses 122 and 123. In the VIPA 124, the incident light is subjected to multiple reflections, and is branched for each of different wavelengths to be emitted. The emitted light is sent to the mirror assembly 110 via the collimator lens 125. The light branched for each wavelength in the VIPA assembly 120, is reflected by the aspherical mirror 112 in the mirror assembly 110, to be again input to the VIPA assembly 120, and is propagated through the VIPA assembly 120 in a direction opposite to the above propagation direction is collected by the optical fiber 121. The VIPA-VDC of such a construction has the feature in that a compensation amount for wavelength dispersion can be changed by moving the aspherical mirror 112 according to a dispersion amount. The reflecting surface of the aspherical mirror 112 is prepared with high accurately so that a desired compensation amount can be obtained.
However, if the mirror 101 is fixed to the jig 102 by adhesive or mechanical fixing as with the above described conventional technique, there is a problem of stress distortion developing on the mirror surface due to the fixing.
More specifically, when for example, as shown in FIG. 12, the mirror 101 is fixed to the jig 102 with adhesive 104, then considering the case of a change in temperature, the shape of the mirror surface is deformed to become concave or convex as shown at the right of FIG. 12, since the material used for the jig 102 has the coefficient of thermal expansion different from that of the mirror 101 (plastic for instance). Moreover, the surface shape of the mirror 101 is sometimes deformed due to shrinkage stress of the adhesive, other than the temperature change. If such distortion develops on the surface of the mirror 101, the surface of the mirror 101 has a curved surface shape different from the designed value and hence a desired compensation amount cannot be obtained in devices such as the aforementioned VIPA-VDC illustrated in FIG. 11. As another drawback of the fixing method using an adhesive, there is a possibility that the mirror 101 comes away from the jig 102 when the adhesive is deteriorated. Especially, for the various types of devices used in optical communication systems, such as the VIPA-VDC, long-term reliability with a product life of 25 years or so is required. Therefore, there is a need to realize a stable fixing method without the use of an adhesive.
In the case where a mechanical fixing method is applied, then for example as shown in FIG. 13, a pressing force acts on a part “b” where the mirror 101 and a member 102B that forces the mirror 101 to a member 102A being a part of the jig, are in contact with each other. However, for a part “a” where there is not contact, since a pressing force does not act (refer to the cross-section A—A in the lower part of FIG. 13), then if the temperature rises, the part “a” of the mirror 101 expands, whereas the part “b” does not expand so much. Consequently, the surface of the mirror 101 is distorted in a convex shape.